Moisture or water and certain other polar hydrocarbon compounds are known poisons for catalysts employed in polymerization processes. For a catalyst to effect polymerization, moisture (or water) and certain other polar hydrocarbon compounds need to be reduced to low levels, typically, less than 20 ppmv, preferably about 1 to 5 ppmv, prior to commencing polymerization. Even low levels of moisture or certain other polar compounds can contribute to static electricity, which in turn can result in fused polymer sheet and skin formations, sometimes causing the reactor to be shut down. For these reasons, it is necessary to provide effective ways of drying the reaction system, or removing polar compounds, such as before startup, during an upset, and after an emergency shutdown in which moisture or other polar compounds may have entered the system.
Commercially, two methods are routinely employed to dry a reaction system--pressure purging or flow purging using a dry gas, hereby referred to as purge gas. The purge gas can include, but is not limited to, air, light hydrocarbons (alkanes having 1-8 carbon atoms, e.g. methane, ethane, butane, isopentane, and the like, especially mixtures thereof), or nitrogen as the drying medium. Preferably the purge gas is nitrogen. During pressure purging, the reaction system is pressured to a high pressure, typically 140 to 180 psig. This is followed by venting to near atmospheric pressure, typically 5 to 10 psig, while maintaining hot (75 to 125 degrees C) recycle gas flow. The system continues in this manner until moisture and/or other contaminants reach the desired low level. Usually, pressure purging is performed before charging a seed bed of polymer to the reactor and after the seed bed is charged to the reactor. Before charging a seed bed to the reactor, the reaction system is dried to a pre-determined ppmv level of water. Once the seed bed is charged, the system is further dried to the same or a lower level of moisture and/or contaminant content.
Flow purging is similar to pressure purging or cycling, except that the pressure is held constant, for example, at about 10 to 20 psig, and a once through flow of about 0.1 fps of dry gas is typically maintained.
In either case, the dry-down time can take hours to several days, often more than one day. The total purge gas consumption is generally greater in approximate proportion to the dry-down time required for conventional purging. Thus, there is a need to reduce the purge gas consumption and reactor downtime required for removal of moisture or other polar hydrocarbon compounds to low ppmv levels, thereby providing a more cost-effective operation.